Multiple bit chalcogenide storage device

ABSTRACT

Multi-terminal chalcogenide memory cells having multiple binary or non-binary bit storage capacity and methods of programming same. The memory cells include a pore region containing a chalcogenide material along with three or more electrical terminals in electrical communication therewith. The configuration of terminals delineates spatially distinct regions of chalcogenide material that may be selectively and independently programmed to provide multibit storage. The application of an electrical signal (e.g. electrical current or voltage pulse) between a pair of terminals effects a structural transformation in one of the spatially distinct portions of chalcogenide material. Application of electrical signals to different pairs of terminals within a chalcogenide device effects structural transformations in different portions of the chalcogenide material. The structural states produced by the structural transformations may be used for storage of information values in a binary or non-binary (e.g. multilevel) system. The selection of terminals provides for the selective programming of specific and distinct portions within a continuous volume of chalcogenide material, where each selectively programmed portion provides for the storage of a single binary or non-binary bit. In devices having three or more terminals, two or more selectively programmable portions are present within the volume of chalcogenide material occupying the pore region and multibit storage is accordingly realized. The instant invention further includes methods of programming chalcogenide memory cells having three or more terminals directed at the storage of multiple bits of information in binary or non-binary systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/657,285, filedSep. 8, 2003, the disclosure of which is herein incorporated byreference.

FIELD OF INVENTION

This invention relates to chalcogenide storage devices. Moreparticularly, this invention relates to devices having multipleterminals in electrical communication with a chalcogenide material inwhich different pairs of terminals are capable of programming differentportions of the chalcogenide material. Most particularly, this inventionrelates to chalcogenide devices that provide a multiple bit data storagecapability.

BACKGROUND OF THE INVENTION

Computers and consumer electronics are critically dependent on thememories used to store and process information. Various types ofmemories, including ROM, RAM, DRAM, SRAM and flash, underlie the storageand processing capabilities of computers and consumer electronics. Thesedifferent forms of memory differ with respect to speed and volatilityand are optimized with respect to specific tasks to provide forefficient operation. ROM, Read Only Memory, is memory that storesprograms used by computers on booting (BIOS program) and fordiagnostics. Data is pre-recorded on ROM and once recorded, data cannotbe removed from ROM, but rather can only be read. ROM is constructedfrom logic hard-wired in silicon. For this reason, ROM is a highlypermanent form of memory that offers a high degree of security and isnot susceptible to attacks by viruses. ROM is a non-volatile form ofmemory, which means that ROM retains its contents when the power isturned off. Most computers have only a few kilobytes of ROM. ROM is alsowidely used in calculators and in peripheral devices such as laserprinters. Variations of ROM include PROM (Programmable ROM), EPROM(Erasable PROM) and EEPROM (Electrically Erasable PROM). PROM is a formof ROM that is produced in an unrecorded state and is once writable andnot erasable. PROM offers purchasers the ability to record programs on aROM medium and the flexibility of changing the program as therequirements of a particular application change. EPROM a form of ROMthat is erasable under action of ultraviolet light and that can bereprogrammed. EEPROM is an electrically erasable form of ROM that can beerased through software.

RAM, Random Access Memory, is the most common type of memory found incomputers and other devices. RAM is the working memory of computers andis the memory utilized by programs. Data can be written, erased andre-recorded on RAM. RAM is a volatile form of memory, which means thatits contents are erased when the power is turned off. DRAM (dynamic RAM)and SRAM (static RAM) are the two most common variations of RAM. DRAM isless expensive, but slower than SRAM and is characterized by a need forconstant refreshing in order to retain data. The refresh requirement ofDRAM is a consequence of the mechanism of data storage in a DRAM memorycell. A DRAM memory cell includes a capacitor and data is stored ascharge on the capacitor. The capacitor charge is not stored permanently,however, and shows a tendency to leak to the substrate on which the DRAMcell is located or to neighboring devices on a chip. Since leakage ofcharge corresponds to loss of data, the charge is periodicallyrefreshed. The refreshing requirement drives up power consumption andunderlies the volatile characteristic of DRAM. Access times for DRAM aretypically on the order of 60 nsec.

SRAM is a form of RAM that retains its information content without theneed to refresh as long as power is maintained. SRAM is faster thanDRAM, but also more expensive because an SRAM memory cell requires moretransistors than DRAM (typically 4-6 transistors as opposed to one forDRAM) and requires more space on a chip. Access times for SRAM are onthe order of 10 nsec and SRAM also has a much shorter cycle time (timebetween successive memory accesses) than DRAM because DRAM requires apause between successive accesses due to refreshing limitations. From aperformance basis, SRAM is superior to DRAM. But given its higher cost,however, it is primarily used in cache memory applications, where highspeed is essential.

Flash memory is the leading non-volatile memory used in consumerelectronics. Flash is a compact form of memory that is portable andconveniently interfaced with many devices. Flash is an erasable andrewritable form of memory. Flash is the memory of choice for many moderndevices including cellphones (where flash is used to store theinstructions needed to send and receive calls as well as to retain phonenumbers), personal digital assistants (where flash is used to storeaddresses, calendar entries, memos etc.) and digital cameras (whereflash is the type of memory used in the erasable media cards that storepictures).

Flash memory is a type of EEPROM that relies on a floating gate to storecharge. The flash memory cell is similar in construction to a transistorand includes a floating gate (typically a polysilicon layer) and atunnel oxide layer that are inserted between the oxide of the controlgate and the channel region of a transistor. Data in the form of a “0”or “1” is stored by controlling the charge of the floating gate. If nocharge is stored on the floating gate, current flows from source todrain when a voltage is applied to the control gate as the transistorturns on. If a charge is stored, current is inhibited and application ofa gate voltage fails to turn on the transistor. The information contentin a flash memory cell is thus determined through a simple read protocolinvolving a determination of whether an applied gate voltage turns thetransistor on.

As the demands for faster, less expensive, smaller and more efficientcomputers and consumer electronics become more stringent, chipmanufacturers and device designers have come to recognize thedeficiencies of current memory technologies and have begun to search foralternative materials and devices for storing data. There is currently agreat deal of interest in identifying replacements for flash memorybecause of the importance of flash memory for many applications andbecause of several shortcomings that have been identified with flashmemory. Current flash memory suffers from two important drawbacks.First, the write time of flash memory is slow (on the order of amicrosecond) and limits the range of applications for flash memory.While suitable for archival storage, flash is unsuitable for use as aworking memory because competitive data processing requires fast writingtimes. Second, the lifetime of flash memory is relatively short as thereliability of data storage in flash memory diminishes after a fewhundred thousand write-erase cycles.

Three new technologies directed at obtaining a faster, more reliablereplacement for flash memory are currently under development. In onetechnology, FRAM (Ferroelectric RAM), a ferroelectric material is usedto store data. An FRAM memory cell includes a capacitor containing aferroelectric material such as PZT that records binary information basedon the orientation of the ferroelectric domains of the ferroelectricmaterial. The ferroelectric domains can be reversible aligned in one oftwo directions to define two binary states that can be distinguished ina read operation based on determining a current upon application of ashort voltage pulse to the capacitor. Depending on the orientation ofthe ferroelectric domains relative to the electric field associated withthe voltage pulse, the current induced by the voltage pulse is eitherhigh or low. A second technology, MRAM (Magnetoresistive RAM), utilizesthe ferromagnetic properties of atoms. MRAM is a magnetic analogue ofFRAM that relies on the ferromagnetic characteristics of a ferromagneticmaterial to store information. A ferromagnetic material includes domainshaving a magnetic dipole, where the domains can be aligned and orientedunder the action of an external magnetic field. As in FRAM, theorientation of aligned magnetic domains defines two binary states thatare used to record information. In one device configuration, MRAMincludes a magnetic tunnel junction that includes two ferromagneticlayers separated by a tunnel oxide where the relative orientation of themagnetic domains of the two ferromagnetic layers dictates that currentflow across the junction. The current flow is high when the twoferromagnetic layers have parallel magnetic domains and is low when thetwo ferromagnetic layers have anti-parallel domains. A third technologywith the potential to replace flash memory is Ovonic Unified Memory(OUM). OUM records information through the phase of a chalcogenide phasechange material. Chalcogenide phase change materials can be reversiblytransformed between amorphous and crystalline states where each statemay correspond to a different binary state. Since the amorphous andcrystalline states differ in resistance by two or more orders ofmagnitude, the two states are readily distinguishable.

FRAM, MRAM and OUM all address the deficiencies of conventional flashmemory. All three potential flash replacement technologies offer fastwriting times and essentially endless cycle life stability. All threetechnologies also are non-volatile and require no static power.Development work in the three flash replacement technologies is focusingon cost, deposition and manufacturing issues. Of greatest concern is theability to integrate the technologies into existing CMOS fabricationprocesses. Also of concern is the development of adequate, reliable andreproducible growth methods for forming uniform thin film layers offerroelectric, ferromagnetic or chalcogenide materials and thecompatibility of these layers with conventional silicon based materials.

An additional consideration concerns the data storage capacity ofpotential flash replacement technologies. Current efforts in thedevelopment of FRAM, MRAM and OUM have focused on memory cells capableof storing one bit of information per memory cell or volume of activeferroelectric, ferromagnetic or chalcogenide material. Under thisassumption, it is believed that development of the three replacementflash technologies will ultimately lead to data storage capacities thatare comparable to those of conventional flash. In this view, it isbelieved that the advantageous writing speed and reliability features ofFRAM, MRAM or OUM will ultimately prevail and convince industry to dropflash and adopt a superior replacement technology. Recent advances inconventional flash memory, however, have raised the entry barrier for areplacement flash technology. These advances have led to the developmentof flash memory that can store two or more bits of information per datacell. As a result, the cost per stored bit of information has droppedconsiderably in current flash technology and the performance and coststandards for a competing replacement technology have increasedcommensurately.

At this point in time, it is unknown whether any of the three currentlyidentified replacement flash technologies will prove to be better thanthe others and whether any of them will perform well enough to displaceconventional flash technology. It is clear, however, that anyreplacement for flash must provide a competitive storage capacity. Aneed exists, therefore, for a non-volatile memory technology capable ofproviding a data storage capacity comparable to the two or more bit percell storage offered by today's flash technology.

SUMMARY OF THE INVENTION

The instant invention provides a chalcogenide device capable ofproviding two or more bits of storage in a volume of chalcogenidematerial contained in the pore region of the device. The chalcogenidematerial has a plurality of structural states and data storage isaccomplished by associating different structural states of thechalcogenide material with different information values in a binary ornon-binary system. Programming or the storage of data occurs by applyingenergy to a chalcogenide material through electrical terminals in anamount sufficient to induce a transformation of the chalcogenidematerial to the structural state associated with the desired storedinformation value.

The instant chalcogenide device includes a volume of chalcogenidematerial with three or more electrical terminals in electricalcommunication therewith. The positioning of the electrical terminals hasthe effect of dividing the volume of chalcogenide material into distinctportions, each of which, in one embodiment, may be independentlyprogrammed to store a bit of information. Each of two or more separateportions of chalcogenide material may be programmed by one or more pairsof electrical terminals and may be independently programmed by at leastone pair of electrical terminals. The separately programmable portionsof chalcogenide material are portions within a larger continuous volumeof a chalcogenide material occupying the pore region of the device. Thedifferent separately programmable portions may comprise the samechalcogenide material composition or may include different chalcogenidematerial compositions.

In one embodiment of the instant invention, selective programming ofindividual portions of chalcogenide material within a larger volume ofchalcogenide material having one or more compositions is provided. Inthis embodiment, a programming pulse applied between a pair ofelectrical terminals of a device induces a structural transformation ina portion of chalcogenide material between that pair of electricalterminals without inducing a structural transformation of chalcogenidematerial in the vicinity of other terminals. In this embodiment, datastorage in one portion of a chalcogenide material occurs withoutdisturbing the information content of an adjacent portion ofchalcogenide material.

In another embodiment of the instant invention, the energy content of aprogramming pulse is in excess of the energy required to transform thestructural state of the chalcogenide material between the two terminalsto which the programming pulse is applied. In this embodiment, theexcess energy may diffuse or otherwise dissipate to one or more adjacentportions of chalcogenide material and may induce a structuraltransformation in those portions to effect a modification in theinformation value stored in those portions.

Multiple bit data storage in the instant memory cell is achieved byapplying electrical signals selectively to electrical terminals thatinfluence separate portions of the chalcogenide material. One bit ofdata may be stored in each selectively programmable portion ofchalcogenide material by applying an electrical signal through or acrossa pair of electrical terminals in electrical communication with thatportion of chalcogenide material. Application of an electrical signal toselected electrical terminals transforms a selected portion ofchalcogenide material to a structural state corresponding to aninformation value intended to be stored. Selective application ofelectrical signals to different pairs of electrical terminals enablesmultibit storage in two or more portions of chalcogenide material in theinstant device. Multiple binary or non-binary bit storage is possiblewith the instant device. In a binary embodiment, each of two or moreportions of a chalcogenide material stores a binary bit of informationand is programmed to one of two structural states. In a non-binaryembodiment, each compartment stores a multary (non-binary) bit ofinformation and is programmed to one of a plurality of structuralstates. The instant invention further comprises an array of multibitchalcogenide data storage cells.

The instant invention further provides a method of programming achalcogenide device to achieve a storage capacity of two or more bits ina volume of chalcogenide material in the pore region of a device. Theprogramming methods include providing sufficient energy in the form ofan electrical pulse applied between a pair of electrical terminals toinduce a transformation in the structural state of a chalcogenide andrepeating for each pair of terminals in a multi-terminal device toachieve information storage or programming of two or moredistinguishable portions of chalcogenide material in a continuous volumeof chalcogenide material within the pore of a device.

For a better understanding of the instant invention, together with otherand further objects thereof, reference is made to the followingdescription, taken in conjunction with the accompanying drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Representative dependence of the electrical resistance of achalcogenide material as a function of energy or current.

FIG. 2. Schematic depiction of a two-terminal chalcogenide storagedevice.

FIG. 3. Schematic depiction of a three-terminal chalcogenide storagedevice.

FIG. 4. One embodiment of a three-terminal storage device according tothe instant invention.

FIG. 5. Schematic depiction of a four-terminal chalcogenide storagedevice.

FIG. 6. Schematic depiction of a three-terminal storage device having anon-uniformly cylindrical pore region.

DETAILED DESCRIPTION OF EMBODIMENTS

The instant invention provides a memory cell capable of multibit datastorage and a method for programming a memory cell to store two or morebits of information. The memory cell of the instant invention includes avolume of chalcogenide material in electrical communication with threeor more electrical terminals where electrical energy provided to theelectrical terminals is used to program the chalcogenide material tostore data. The chalcogenide material is the data storage medium of theinstant memory cell. The chalcogenide material is a phase changematerial that is capable of reversibly transforming among a plurality ofstructural states by providing electrical energy in the form of anelectrical current or voltage pulse. The different structural states ofthe phase change material are distinguishable on the basis of acharacteristic property such as electrical resistance and each can beuniquely associated with different information states to provide thebasis of a data storage protocol.

The chalcogenide materials of the instant memory cells have beenpreviously utilized in optical and electrical memory and switchingapplications and some representative compositions and properties havebeen discussed in in U.S. Pat. Nos. 5,543,737; 5,694,146; 5,757,446;5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; and 6,087,674;the disclosures of which are incorporated by reference herein, as wellas in several journal articles including “Reversible ElectricalSwitching Phenomena in Disordered Structures”, Physical Review Letters,vol. 21, p. 1450-1453 (1969) by S. R. Ovshinsky; “AmorphousSemiconductors for Switching, Memory, and Imaging Applications”, IEEETransactions on Electron Devices, vol. ED-20, p. 91-105 (1973) by S. R.Ovshinsky and H. Fritzsche; the disclosures of which are incorporated byreference herein. General characteristics and comments about phasechange chalcogenide materials are reviewed in the context of the instantinvention in the following discussion.

Representative chalcogenide materials suitable for use in the instantinvention are those that include one or more elements from column VI ofthe periodic table (the chalcogen elements) and optionally one or morechemical modifiers from columns III. IV or V. One or more of S, Se, andTe are the most common chalcogen elements included in the chalcogenidedata storage material of the instant memory devices. Materials thatinclude Ge, Sb, and/or Te, such as Ge₂Sb₂Te₅, are examples ofchalcogenide materials in accordance with the instant invention. Thechalcogen elements are characterized by divalent bonding and thepresence of lone pair electrons. The divalent bonding leads to theformation of chain and ring structures upon combining chalcogen elementsto form chalcogenide materials and the lone pair electrons provide asource of electrons for forming a conducting filament in switchingapplications. The conducting filament may also contribute to or aid indriving phase changes that occur between different structural states.Trivalent and tetravalent modifiers such as Al, Ga, In, Ge, Sn, Si, P,As and Sb enter the chain and ring structures of chalcogen elements andprovide points for branching and crosslinking. The structural rigidityof chalcogenide materials depends on the extent of crosslinking andinfluences their ability to undergo crystallization or other structuraltransformations or rearrangements.

An important feature of the chalcogenide materials in the context of theinstant invention is their ability to undergo a reversible phasetransformation between or among two or more structural states. Thechalcogenide materials have structural states that include a crystallinestate, one or more partially crystalline states and an amorphous state.The crystalline state may be a single crystalline state or apolycrystalline state. As used herein, a partially crystalline staterefers to a structural state of a volume of chalcogenide material thatincludes an amorphous portion and a crystalline portion. Preferably, aplurality of partially crystalline states exists for the phase changematerial that may be distinguished on the basis of the relativeproportion of the amorphous and crystalline portions. Fractionalcrystallinity is one way to characterize the structural states of achalcogenide phase change material. The fractional crystallinity of thecrystalline state is 100%, the fractional crystallinity of the amorphousstate is 0%, and the partially crystalline states have fractionalcrystallinities that vary continuously between 0% (the amorphous limit)and 100% (the crystalline limit). Phase change chalcogenide materialsare thus able to reversibly transform among a plurality of structuralstates that vary inclusively between fractional crystallinities of 0%and 100%.

The ability and facility of a chalcogenide material to undergostructural transformations among structural states having variousfractional crystallinities depends on the composition and structuralcharacteristics of the chalcogenide material. More highly crosslinkedchalcogenide materials are more structurally rigid and generally includea higher concentration of modifiers. The more highly crosslinkedchalcogenide materials are more difficult to crystallize because theatomic rearrangements required to nucleate and grow a crystalline phaseare inhibited due to the rigidity of the structure. More lightlycrosslinked chalcogenide materials more readily undergo full or partialcrystallization.

Transformations among the structural states of a chalcogenide materialare induced by providing energy to the chalcogenide material. Energy invarious forms can influence the fractional crystallinity of achalcogenide material and hence, induce structural transformations.Suitable forms of energy include electrical energy, thermal energy,optical energy or other forms of energy that induce electrical, thermalor optical effects in a chalcogenide material (e.g. particle beamenergy) or combinations of the foregoing forms of energy. Continuous andreversible variability of the fractional crystallinity is achievable bycontrolling the energy environment of a chalcogenide material. Acrystalline state can be transformed to a partially crystalline or anamorphous state, a partially crystalline state can be transformed to acrystalline or amorphous state, and an amorphous state can betransformed to a partially crystalline or crystalline state throughproper control of the energy environment of a chalcogenide material.Some considerations associated with the use of thermal, electrical andoptical energy to induce structural transformations are presented in thefollowing discussion.

The use of thermal energy to induce structural transformations exploitsthe thermodynamics and kinetics associated with the crystalline toamorphous or amorphous to crystalline phase transitions. An amorphousphase may be formed, for example, from a partially crystalline orcrystalline state by heating a chalcogenide material above its meltingtemperature and cooling at a rate sufficient to inhibit the formation ofcrystalline phases. A crystalline phase may be formed from an amorphousor partially crystalline state, for example, by heating a chalcogenidematerial above the crystallization temperature for a sufficient periodof time to effect nucleation and/or growth of crystalline domains. Thecrystallization temperature is below the melting temperature andcorresponds to the minimum temperature at which crystallization mayoccur. The driving force for crystallization is typically thermodynamicin that the free energy of a crystalline or partially crystalline stateis lower than the free energy of an amorphous state so that the overallenergy of a chalcogenide material decreases as the fractionalcrystallinity increases. Formation (nucleation and growth) of acrystalline state or crystalline domains within a partially crystallinestate is kinetically inhibited, however, so that heating below themelting point promotes crystallization by providing energy thatfacilitates the rearrangements of atoms needed to form a crystallinephase or domain. The fractional crystallinity of a partially crystallinestate can be controlled by controlling the temperature or time ofheating of crystalline or partially crystalline state or by controllingthe temperature or rate of cooling of an amorphous or partiallycrystalline state.

The use of optical energy to induce structural transformation involvesproviding energy in the form of an optical pulse to a chalcogenidematerial. The optical pulse may be from a laser or a conventional lampor incandescent source. The optical pulse contains optical energy thatis transferred to a chalcogenide material when an optical beam isdirected at or otherwise incident to a chalcogenide material. Opticalenergy can induce transformations among the crystalline, partiallycrystalline and amorphous states of a chalcogenide material. Use ofchalcogenide materials in CD and DVD applications, for example, is wellknown and relies on the use of an optical source to inducetransformations between the crystalline and amorphous states of achalcogenide material. Frequently an unrecorded state corresponds to thecrystalline state and writing or storage of data occurs through theselective formation of amorphous marks where the pattern of markscorresponds to stored information. Transfer of optical energy to achalcogenide material may occur through optical absorption and may beaccompanied by a thermal effect associated with the dissipation ofoptical energy within a chalcogenide material. Optical energy may beused to melt or crystallize a chalcogenide material. By controlling thewavelength, intensity, duration, and profile of optical pulses, it ispossible to manage the energy environment of a chalcogenide material andto reversible transform a chalcogenide material among its crystalline,partially crystalline and amorphous states.

The use of electrical energy to induce structural transformations relieson the application of electrical (current or voltage) pulses to achalcogenide material. The mechanism of electrically induced structuraltransformations may be electronic in nature, possibly with anaccompanying or consequent thermal contribution. By controlling themagnitude and/or duration of electrical pulses applied to a chalcogenidematerial, it is possible to vary continuously vary the fractionalcrystallinity. The influence of electrical energy on the structure of achalcogenide material is frequently depicted in terms of the variationof the electrical resistance of a chalcogenide material with the amountof electrical energy provided or the magnitude of the current or voltagepulse applied to a chalcogenide material. A representative depiction ofthe electrical resistance (R) of a chalcogenide material as a functionof electrical energy or current pulse magnitude (Energy/Current) ispresented in FIG. 1 herein. FIG. 1 shows the variation of the electricalresistance of a chalcogenide material with electrical energy or currentpulse magnitude and may generally be referred to as a resistance plot.

The resistance plot includes two characteristic response regimes of achalcogenide material to electrical energy. The regimes areapproximately demarcated with the vertical dashed line 10 shown inFIG. 1. The regime to the left of the line 10 may be referred to as theaccumulating regime of the chalcogenide material. The accumulationregime is distinguished by a nearly constant or weakly varyingelectrical resistance with increasing electrical energy that culminatesin an abrupt decrease in resistance beyond a threshold energy. Theaccumulation regime thus extends, in the direction of increasing energy,from the leftmost point 20 of the resistance plot, through a plateauregion (generally depicted by 30) corresponding to the range of pointsover which the resistance variation is small or negligible to the setpoint or state 40 that follows an abrupt decrease in electricalresistance. The left side of the resistance plot is referred to as theaccumulating regime because the structural state of the chalcogenidematerial continuously evolves as energy is applied with the fractionalcrystallinity of the structural state correlating with the totalaccumulation of energy. The leftmost point 20 corresponds to thestructural state in the accumulating regime having the lowest fractionalcrystallinity. As energy is added, the fractional crystallinityincreases and the chalcogenide material transforms in the direction ofincreasing energy among a plurality of partially crystalline statesalong the plateau 30. Selected accumulation states (structural states inthe accumulation region) are marked with squares in FIG. 1. Uponaccumulation of a threshold amount of energy, the fractionalcrystallinity of the chalcogenide material increases sufficiently toeffect a setting transformation characterized by a dramatic decrease inelectrical resistance and stabilization of the set state 40. Thestructural states in the accumulation regime may be referred to asaccumulation states of the chalcogenide material. Structuraltransformations in the accumulating regime are unidirectional in thesense that they progress in the direction of increasing energy withinthe plateau region 30 and are reversible only by first driving thechalcogenide material through the set point 40 and resetting asdescribed in, for example U.S. patent application Ser. Nos. 10/155,527and 10/189,749, the disclosures of which are incorporated by referenceherein.

While not wishing to be bound by theory, the instant inventors believethat the addition of energy to a chalcogenide material in theaccumulating regime leads to an increase in fractional crystallinitythrough the nucleation of new crystalline domains, growth of existingcrystalline domains or a combination thereof. It is believed that theelectrical resistance varies only weakly along the plateau 30 despitethe increase in fractional crystallinity because the crystalline domainsform or grow in relative isolation of each other so as to prevent theformation of a contiguous crystalline network that spans thechalcogenide material. This type of crystallization may be referred toas sub-percolation crystallization. The setting transformation coincideswith a percolation threshold in which a contiguous, interconnectedcrystalline network forms within the chalcogenide material. Such anetwork may form, for example, when crystalline domains increasesufficiently in size to impinge or overlap with neighboring domains.Since the crystalline phase of chalcogenide materials is more conductiveand less resistive than the amorphous phase, the percolation thresholdcorresponds to the formation of a contiguous low resistance conductivepathway through the chalcogenide material. As a result, the percolationthreshold is marked by a dramatic decrease in the resistance of thechalcogenide material. The leftmost point of the accumulation regime maybe an amorphous state or a partially crystalline state lacking acontiguous crystalline network. Sub-percolation crystallizationcommences with an initial amorphous or partially crystalline state andprogresses through a plurality of partially crystalline state havingincreasingly higher fractional crystallinities until the percolationthreshold and setting transformation occur. Further discussion of thebehavior of chalcogenide materials in the accumulation regime isprovided in U.S. patent application Ser. Nos. 10/144,319; 10/155,527 and10/189,749 and in U.S. Pat. Nos. 5,912,839 and 6,141,241; thedisclosures of which are incorporated by reference herein.

The regime to the right of the line 10 of FIG. 1 may be referred to asthe greyscale regime or greyscale region. The greyscale regime extendsfrom the set state 40 through a plurality of intermediate states(generally depicted by 50) to a reset point or state 60. The variouspoints in the greyscale regime may be referred to as greyscale states ofthe chalcogenide material. Selected greyscale states are marked withcircles in FIG. 1. Structural transformations in the greyscale regimemay be induced by applying an electric current or voltage pulse to achalcogenide material. In FIG. 1, an electric current pulse isindicated. In the greyscale regime, the resistance of the chalcogenidematerial varies with the magnitude of the applied electric pulse. Theresistance of a particular state in the greyscale regime ischaracteristic of the structural state of the chalcogenide material andthe structural state of a chalcogenide material is dictated by themagnitude of the current pulse applied in the greyscale region. Thefractional crystallinity of the chalcogenide material decreases as themagnitude of the current pulse increases. The fractional crystallinityis highest for greyscale states at or near the set point 40 andprogressively decreases as the reset state 60 is approached. Thechalcogenide material transforms from a structural state possessing acontiguous crystalline network at the set state 40 to a structural statethat is amorphous or substantially amorphous or partially crystallinewithout a contiguous crystalline network at the reset state 60. Theapplication of current pulses having increasing magnitude has the effectof converting portions of the crystalline network into an amorphousphase and leads to a disruption or interruption of contiguous highconductivity crystalline pathways in the chalcogenide material. As aresult, the resistance of the chalcogenide material increases as themagnitude of an applied current pulse increases in the greyscale region.

In contrast to the accumulating region, structural transformations thatoccur in the greyscale region are reversible and bi-directional. Theresponse of a chalcogenide material to a current pulse is determined bythe magnitude of the current pulse relative to the magnitude of thecurrent pulse associated with the initial state of the chalcogenidematerial at the time the current pulse is applied. As indicatedhereinabove, each state in the greyscale region may be identified by itsresistance and a current pulse magnitude where application of thatcurrent pulse magnitude induces changes in fractional crystallinity thatproduce the particular resistance value of the state. Application of asubsequent current pulse may increase or decrease the fractionalcrystallinity relative to the fractional crystallinity of the initialstate of the chalcogenide material. If the subsequent current pulse hasa higher magnitude than the pulse used to establish the initial state,the fractional crystallinity of the chalcogenide material decreases andthe structural state is transformed from the initial state in thedirection of the reset state along the greyscale resistance curve.Similarly, if the subsequent current pulse has a lower magnitude thanthe pulse used to establish the initial state, the fractionalcrystallinity of the chalcogenide material increases and the structuralstate is transformed from the initial state in the direction of the setstate along the greyscale resistance curve. Further discussion of theproperties of chalcogenide materials in the greyscale region may befound, for example, in U.S. Pat. Nos. 5,296,716 and 5,414,271; thedisclosures of which are incorporated by reference herein.

As described more fully hereinbelow, data is stored in chalcogenidematerials through the structural states of a chalcogenide material. Thestorage of data occurs through a structural transformation of thechalcogenide material to a structural state corresponding to theinformation value being stored. The storage of data through appropriatestructural transformations may be referred to herein as programming thechalcogenide material. Since information is encoded as structural statesof a chalcogenide material in the instant invention, the reading of datarequires a detection or determination of the structural state of achalcogenide material. A method for reading structural states in theaccumulation region has been discussed in, for example, U.S. patentapplication Ser. Nos. 10/144,319; 10/155,527 and 10/189,749, and mayinclude a determination of the amount of energy required to transform anaccumulation state to the set state. This amount of energy is one way touniquely characterize or distinguish structural states in theaccumulation region. The reading of structural states in the greyscaleregion is most commonly done through a resistance measurement. Theelectrical resistance of structural states in the greyscale regionprovides an effective metric for distinguishing greyscale states.

In the instant invention, a device and method are provided wherein achalcogenide material is used for data storage where data is storedthrough the structural states of the chalcogenide material. Distinctstructural states of a chalcogenide material are assigned to specificdata values and storage of a data value occurs by transforming thechalcogenide material into a structural state characteristic of the datavalue. The data values may also be viewed as information values sinceparticular data values are oftentimes encoded representations ofinformation. In binary data storage, for example, information is encodedby strings of the binary data values “0” and “1” where “0” and “1” maybe viewed as information values in a binary system. The structuralstates of a chalcogenide material used to store information may bereferred to as information states since each distinct structural statecorresponds to a particular information or data value. The informationstates of a chalcogenide material may be selected from the accumulationstates, greyscales states or combination thereof. In a binary datastorage system, for example, “0” and “1” may be viewed as informationvalues and each information value may be assigned to a distinctstructural state of a chalcogenide material. The selected structuralstates of the chalcogenide material may thus be viewed as theinformation states associated with the information values “0” and “1”.

In addition to binary storage, a chalcogenide material may also beconfigured to provide non-binary (multary) storage where information ordata is stored using three or more information values. In a ternary(e.g. base 3) system, for example, data may be encoded using threeinformation values (e.g. “0”, “1” and “2”) and each information valuemay be associated with a distinct structural state of a chalcogenidematerial so that three information states are selected from among thestructural states of the chalcogenide. Structural states in theaccumulation region, greyscale region or combination thereof may beselected and associated with particular information values. Storage of aparticular information value is accomplished by transforming thechalcogenide material to the information state associated with theinformation value. As described hereinabove, the transformation may beaccomplished by providing energy (e.g. electrical energy, opticalenergy, particle beam energy, pulsed energy etc.) in an appropriateamount. Storage systems having a higher number of levels (e.g. fourlevel, five level etc. up to an arbitrary number of levels) may beanalogously implemented by selecting an appropriate number ofinformation states from among the structural states of the chalcogenidematerial. Where data is encoded as a string of information values in astorage system having a particular number of levels, the data may bestored by writing each information value in the string to a separateregion of chalcogenide material.

In a binary string, the region of memory material used to store aninformation value is typically referred to as a bit of memory material,or, more simply, a bit. In conventional terms, a bit represents anamount or portion of a binary memory material (normally silicon)allocated for the purpose of storing a “0” or a “1”, where the memorymaterial is capable of storing either binary information value(separately, not simultaneously) and where it is possible to switchbetween the two values through a data programming or processingtechnique. A bit may analogously be defined for a chalcogenide datastorage medium utilizing a binary storage system. In the instantinvention, a bit may further be generalized to multary (multiple valued,multilevel or non-binary) data storage systems. In a multary system, abit corresponds to a portion or region of memory material (chalcogenide,silicon or otherwise) allocated for the storage of an information valueand having the capability of being switched among the differentinformation values associated with the storage system. In a four level(e.g. base 4) storage system, for example, the information values “0”,“1”, “2”, and “3” may be used and information states may be selectedfrom among the structural states of a chalcogenide material for each ofthese information values. In this four level system in the context ofthe instant invention, a bit corresponds to a portion or region ofchalcogenide material allocated for the storage of one of the fourinformation values where switching between information values ispossible by providing energy as described hereinabove and in the matterincorporated by reference to induce a structural transformation from aninformation state associated with one information value to aninformation state associated with a different information value. A bitmay be analogously described for an arbitrary multiple level datastorage system. As used herein, a bit thus corresponds to a portion orvolume of memory material allocated for the storage of one informationvalue of a storage system having any number of levels.

In conventional data storage devices, each bit of memory material isincluded in a separate memory cell having two electrical terminals wherethe two terminals are used to address and program the memory cell tostore a particular information value. Two terminal memory cells forstoring binary bits in silicon are well known in the art. Two terminalmemory cells utilizing a chalcogenide material as the data storagemedium have also been discussed. U.S. Pat. Nos. RE37,259; 5,414,271; and5,296,716, for example, describe single cell chalcogenide memoryelements in which a chalcogenide material is placed between twoelectrical terminals and programmed to particular memory statestherewith. The memory states are selected from the greyscale states ofthe chalcogenide material. U.S. Pat. No. 5,912,839 further describes asingle cell memory element having two electrical terminals and achalcogenide material disposed therebetween where the memory states areselected from among the accumulation states of the chalcogenidematerial.

U.S. Pat. Nos. RE37,259; 5,414,271; 5,296,716; and 5,912,839 furtherdescribe multibit storage in a chalcogenide material where multibitstorage refers to an increase in information storage density through theuse of a chalcogenide material having multiple storage levels. In U.S.Pat. Nos. RE37,259; 5,414,271; 5,296,716; and 5,912,839, multibitstorage is described in terms of binary bits and storage of theequivalent of multiple binary bits of information is accomplished in amemory cell having a single region of chalcogenide material through amultilevel storage system. In these patents, for example, a four levelstorage scheme provides a storage capacity equivalent to that of twobinary bits since two binary bits encompass four different states (00,01, 10, 11). An eight level storage scheme provides a storage capacityequivalent to that of three binary bits etc.

The instant invention provides a memory cell having three or moreelectrical terminals and a method of programming such cells for mulitbitstorage. In contrast to the memory cells of U.S. Pat. Nos. RE37,259;5,414,271; 5,296,716; and 5,912,839, the instant invention providesmultiple bit storage for both binary and non-binary bits. Multiple bitstorage in the context of the instant invention occurs through theselective addressing of specific portions of a region of chalcogenidematerial contained within the pore of a memory cell. The selectiveaddressing is accomplished through inclusion of additional electricalterminals that are placed in electrical communication with the region ofchalcogenide material.

A schematic comparison of a two terminal memory cell and one embodimentof a three terminal memory cell according to the instant invention isprovided in FIGS. 2 and 3. FIG. 2 shows a typical two terminalchalcogenide memory cell. The cell includes pore region filled with achalcogenide material 110 that is in electrical contact with a topelectrical terminal 140 and a bottom electrical terminal 150. The poreregion is preferably cylindrical in shape, but may be non-cylindrical(e.g. rectangular, channel, etc.) as well. Insulating material 170separates the terminals 140 and 150. Data storage in the two terminalcell is accomplished by providing electrical energy across the terminals140 and 150. Typically, electrical energy is provided in the form of acurrent or voltage pulse. Application of electrical energy in anappropriate amount programs or transforms the chalcogenide material tothe structural state corresponding to the information value that onewishes to store.

FIG. 3 depicts an embodiment of a three terminal memory cell accordingto the instant invention. The three terminal memory cell includes a porefilled with a chalcogenide material 210 that is in contact with a topelectrical terminal 240, a bottom electrical terminal 250 and anintermediate electrical terminal 260. An insulating or dielectricmaterial or materials 270 separates the electrical terminals 240, 250and 260. The pore may be cylindrical or non-cylindrical in shape. If thepore is cylindrical, the intermediate terminal 260 is generally annularin shape. If the pore is non-cylindrical, the intermediate terminal 260is generally circumferential in shape. Storage of data in the threeterminal cell occurs by providing an electrical signal to thechalcogenide material through any pair of terminals. Since the deviceincludes three terminals and since structural transformations of thechalcogenide material can be effected by providing electrical energythrough two terminals, the three terminal device provides for multipleoptions for the programming or transformation of the chalcogenidematerial and permits programming of selected portions or regions ofchalcogenide material in the pore of the device. The multiplicity ofprogramming options and ability to program selected portions of thechalcogenide material distinguish the instant multiterminal devices fromthe two terminal devices of the prior art.

In the embodiment of FIG. 3, the three terminals provide for threeprogramming modes. In a one programming mode, an electrical signal (e.g.current pulse, voltage pulse) is provided between the top terminal 240and bottom terminal 250 to provide a data storage capability analogousto that of a two terminal chalcogenide memory cell. In this programmingmode, the entire volume of chalcogenide material 210 in the pore issubject to the influence of the electrical signal. This programming modeprovides for single bit storage in a binary or non-binary system.

In a second programming mode, an electrical signal is provided betweenthe top terminal 240 and the intermediate terminal 260. In thisprogramming mode, the influence of the electrical signal can be limitedto only a portion of the chalcogenide material 210. More specifically,application of an electrical signal between terminals 240 and 260generally influences only that portion of the chalcogenide materiallocated between those terminals while leaving the portion of thechalcogenide material located between intermediate terminal 260 andbottom terminal 250 essentially undisturbed. As a result, a structuraltransformation may be selectively induced in the upper portion 220 ofthe chalcogenide material and not the lower portion 230. Selectiveprogramming of the upper portion 220 of the chalcogenide material 210 inthe pore is thus achievable through the application of an electricalsignal between top terminal 240 and intermediate terminal 260.

In a third programming mode, an electrical signal is provided betweenthe bottom terminal 250 and the intermediate terminal 260. In thisprogramming mode, the influence of the electrical signal can also belimited to only a portion of the chalcogenide material 210. Morespecifically, application of an electrical signal between terminals 250and 260 generally influences only that portion of the chalcogenidematerial located between those terminals while leaving the portion ofthe chalcogenide material located between intermediate terminal 260 andtop terminal 240 essentially undisturbed. As a result, a structuraltransformation may be selectively induced in the lower portion 230 ofthe chalcogenide material and not the upper portion 220. Selectiveprogramming of the lower portion 230 of the chalcogenide material 210 inthe pore is thus achievable through the application of an electricalsignal between bottom terminal 250 and intermediate terminal 260.

Attainment of selective programming requires the application ofelectrical signals of suitable magnitude. More specifically, appliedsignals that provide excess amounts of electrical energy may lead toexcursions of energy to portions of the chalcogenide material for whichprogramming was not intended. At excessively high current or voltagemagnitudes, a signal applied between two terminals may induce excessheating of the chalcogenide material between those two terminals andthis heating may lead to dissipation of thermal energy to portions ofthe chalcogenide material not located between the terminals to which thesignal is applied. Similarly, at sufficiently high current or voltagemagnitudes, a signal applied between two terminals may create aconductive filament that saturates the portion of chalcogenide materialbetween the two terminals. Signal levels above saturation may lead toexpansion of the filament to portions of the chalcogenide materialoutside of the portion between the two terminals to which the signal isapplied and thus produce an electrical effect that may alter thestructural state of the chalcogenide material outside of the region forwhich selective programming is desired.

Electrical or thermal energy excursion effects may extend the range ofinfluence of an electrical signal applied between two terminals toportions of the chalcogenide material beyond the intended range ofinfluence. If sufficient in magnitude, they may extend even so far asthe unutilized electrical terminal and thereby act to defeat or inhibita selective programming effect. In the context of the instant invention,energy excursion from a programmed portion of chalcogenide material toan unprogrammed portion of chalcogenide material may be a deleterious orbeneficial effect depending on the programming objective. In oneembodiment of the instant invention selective programming is desired. Inthis embodiment, the desired outcome of selective programming is anability to influence less than all of the chalcogenide material in thepore through use of more than two electrical terminals. By properlycontrolling the magnitude of the electrical signal applied between twoterminals, excursions of thermal or electrical energy to portions of thechalcogenide material not intended to be programmed can be minimized toprovide substantially selective programming of the portion ofchalcogenide material between the terminals to which the electricalsignal is applied.

In another embodiment of the instant invention, the energy content of aprogramming pulse applied between two terminals is adjusted to provide acontrolled amount of excess energy that may be constructively utilizedto influence portions of the chalcogenide material adjacent to theportion being directly programmed. In this embodiment, a programmingpulse applied between two terminals of a device having three or moreterminals influences portions of the chalcogenide material adjacent tothe portion located between the two terminals to which the programmingpulse is applied. Portions of chalcogenide material, for example in thevicinity of terminals not utilized in the application of a particularprogramming pulse may be influenced and a structural transformationinduced therein. In this embodiment, the excess energy associated with aprogramming pulse may influence the information state of a bit otherthan the one being programmed. By controlling the amount of excessenergy, it is possible to continuously vary the information state of anon-programmed bit and to control the extent of correlation of theinformation state of the programmed bit relative to the informationstate of a non-programmed bit.

As described hereinabove, conventional two terminal chalcogenide memorycells provide single binary bit or single multary bit storage capacityper cell (where single multary bit storage in the context of the instantinvention corresponds to the multibit storage described in the foregoingprior art describing two terminal memory cells). This type of storagefollows from the fact that the electrical signal used in programming atwo terminal cell influences a volume or path of chalcogenide materialthat fully extends between the two terminals. It is not possible, forexample, to selectively program a pathway or volume of chalcogenidematerial that extends from one terminal of a two terminal device to aselected point in the interior of the pore (i.e. to some point notextending to the other terminal) without influencing the portion ofchalcogenide material between the selected point and the other terminalof the device. If, for example, a current pulse having a particularamplitude programs the full length of the pore between the terminals ofa two terminal device (e.g. top terminal 140 and bottom terminal 150 ofthe device shown in FIG. 2), a current pulse having half of thatparticular amplitude does not simply program half of the length of thepore. Halving of the amplitude may produce a pulse that is incapable ofprogramming at all, a pulse that saturates the pore and programs orinfluences the chalcogenide material along the full length of the pore,or a pulse that does not saturate the pore and programs or influencesthe chalcogenide material along the full length of the pore where theparticular effect depends on the particular pulse amplitude used toprogram. A result whereby the pulse influences less than the full lengthof chalcogenide material does not occur and is a consequence of the factthat application of an electrical signal between the two terminals of atwo terminal device necessarily programs or influences a pathway ofchalcogenide material extending fully between the two terminals. Thisrequirement follows from the functionality of chalcogenide materialswhereby formation of a filament serves as a preliminary effect in theprogramming or structural transformations of a chalcogenide material.The filament is a conductive region that extends from one electricalterminal to another electrical terminal and through establishment of afilament, a pathway is created in the chalcogenide material that extendsover the full length of the pore that is sandwiched by the two terminalsof a two terminal memory cell. The presence of the filament creates aregion of influence of the electrical signal that extends from oneterminal to the other terminal of a two terminal device.

Through the instant selective programming, it is possible to selectiveprogram or influence portions of the chalcogenide material that onlyextend between selected pairs of terminals selected from a group ofthree or more terminals in electrical communication with thechalcogenide. A filament can be selectively formed between any pair ofterminals of a multi-terminal device to provide a selective programmingcapability. Selective filament formation may permit the formation of afilament that extends from one electrical terminal to another electricalterminal by providing an electrical signal between those two terminalswhere the filament does not extend to other terminals of the device.Thus, in the example of FIG. 3, it is possible to program or influenceportions of chalcogenide material extending over a length less than thelength between the top terminal 240 and bottom terminal 250 through useof intermediate terminal 260. As described in one embodimenthereinabove, the three terminal embodiment depicted in FIG. 3 permitsthe selective formation of a filament and an influencing of chalcogenidematerial along a pathway extending from intermediate terminal 260 to topterminal 240 without the involvement of bottom terminal 250 and withoutprogramming or influencing portions of the chalcogenide materialadjacent to bottom terminal 250. Chalcogenide material betweenintermediate terminal 260 and bottom terminal 250 may be similarlyprogrammed or influenced without influencing chalcogenide materialadjacent to top terminal 240. Selective programming of portions of avolume of chalcogenide material may thus be achieved through theformation of a conductive filament that extends between selected pairsof terminals without extending to other terminals of the device in oneembodiment of the instant invention.

One embodiment of the instant invention thus provides the ability toindependently program selected portions of a volume of chalcogenidematerial residing in the pore of a memory device. Selective programmingpermits the inducement of structural transformations of a chalcogenidematerial in selected portions or regions of the pore, where the selectedportions are delineated through the configuration or placement ofelectrical terminals in electrical communication with the chalcogenidematerial. Since the selected portions may be independently programmableby providing an electrical signal between the appropriate pair ofelectrical terminals, the instant invention provides for the independentstorage of information values in different spatial regions of a pore ofa memory cell. As a result, the instant invention provides for truemultiple bit data storage. In the three terminal device of FIG. 3, forexample, a first bit of information may be stored in the portion ofchalcogenide material influenced by providing an electrical signalbetween top terminal 240 and intermediate terminal 260 and a second bitof information may be stored in the portion of chalcogenide materialinfluenced by providing an electrical signal between bottom terminal 250and intermediate terminal 260. The two bits of information are thusstored in different portions of chalcogenide material and may be storedsimultaneously within the pore of the memory device. Each of the storedbits may be a binary bit or a multary bit. The two bits may furthercorrespond to information values from the same (e.g. both binary, bothternary etc.) or different (e.g. one binary, one non-binary, one threelevel bit, one four level bit etc.) base systems.

One example of a device structure according to the instant invention isshown in FIG. 4. FIG. 4 shows a cross-sectional view of a three terminaldevice structure. The three terminals are labeled T(1), T(2), and T(3).A plurality of these devices was formed on a 6″ silicon wafer. Thedevices and layers on the wafer were formed using conventionalsputtering, chemical vapor deposition, etching, and lithographytechniques. The structure includes a silicon wafer substrate 310, athermal oxide layer 320, a bottom terminal 330 that includes aconductive layer 340 formed from TiW or a combination or Ti and TiN anda carbon barrier layer 350, an SiO_(x)/SiN_(x) insulating region 360, anintermediate terminal 370 formed from TiW, a pore filled with achalcogenide material 380, a top terminal 390 that includes a carbonbarrier layer 400 and a conductive layer 410 that includes Ti and TiN,and an Al layer 420. In this example, the chalcogenide material 380 isGe₂Te₂Sb₅ and is labeled GST in FIG. 3. The barrier layers inhibitdiffusion and electromigration of material into the chalcogenide regionand improve the cycle life of the device. Typical layer thicknesses areas follows: conductive layer 340 (100 nm), barrier layer 350 (30 nm),intermediate terminal 370 (10-40 nm), barrier layer 400 (100 nm), andconductive layer 410 (100 nm). The pore region occupied by thechalcogenide material in device of this example is cylindrical with aheight of approximately 0.1 micron and a diameter of about 1 micron. Theterminals 330, 370 and 390 are in electrical communication with thechalcogenide. The intermediate terminal 370 circumscribes thechalcogenide material 380. The terminals are separated by an insulatingmaterial so that electrical communication between terminals occursthrough the chalcogenide material.

The device shown in FIG. 4 was tested for selective programmingaccording to the instant invention. In a first experiment, thechalcogenide material of the device was initially transformed to its setstate. In the set state, the resistance between each pair of terminalswas measured. The resistance between the intermediate terminal 370 andthe bottom terminal 330 was measured to be 14 kΩ and the resistancebetween the top terminal 390 and intermediate terminal 370 was measuredto be 6 kΩ. A programming current pulse was subsequently applied betweenthe intermediate terminal 370 and bottom terminal 330. The magnitude andduration of the pulse were 6 mA and 100 ns, respectively. The energy ofthe applied programming pulse was sufficient to induce a structuraltransformation in at least a portion of the chalcogenide materialbetween the intermediate terminal 370 and bottom terminal 330. Afterapplication of the programming pulse, the resistance between each pairof terminals was measured again. The resistance between the intermediateterminal 370 and the bottom terminal 330 was now measured to be 1.2 MΩand the resistance between the top terminal 390 and intermediateterminal 370 was now measured to be 8.5 kΩ. This experiment shows thatapplication of the programming pulse between the intermediate terminal370 and bottom terminal 330 leads to a dramatic increase in theresistance between the intermediate terminal 370 and bottom terminal 330without significantly affecting the resistance between the intermediateterminal 370 and top terminal 390. A structural transformation of thechalcogenide material has been induced in the lower portion of the porewithout influencing the chalcogenide material in the vicinity of the topterminal 390. The high resistance measured between the intermediateterminal 370 and bottom terminal 330 contrasts with the low resistancemeasured between the intermediate terminal 370 and top terminal 390. Thetwo resistance values correspond to different structural states in theportions of chalcogenide material between the different terminal pairsand may be separately identified with a different information value.This example thus demonstrates the storage of two bits of information ina given volume of chalcogenide material.

After application of the pulse between the bottom terminal 330 andintermediate terminal 370 and measurement of the resistances asdescribed above, a second programming pulse was applied to the device.The second pulse had an amplitude of 75 mA and a duration of 100 ns andwas applied between the intermediate terminal 370 and top terminal 390.The magnitude of the programming pulse was sufficient to induce astructural transformation of at least a portion of the chalcogenidematerial between the intermediate terminal 370 and top terminal 390.After application of the programming pulse, the resistance between eachpair of terminals was measured again. The resistance between theintermediate terminal 370 and the bottom terminal 330 after applicationof the second pulse was measured to be 64 kΩ and the resistance betweenthe top terminal 390 and intermediate terminal 370 was now measured tobe 43 kΩ. This experiment shows that application of a programming pulsebetween the intermediate terminal 370 and top terminal 390 leads to anincrease in the resistance of the chalcogenide material between theintermediate terminal 370 and top terminal 390. The higher resistanceindicates that the programming pulse has transformed the structuralstate of the chalcogenide toward a less crystalline state.

In this experiment, in addition, the resistance between the intermediateterminal 370 and bottom terminal 330 was also influenced. Morespecifically, the resistance between the intermediate terminal 370 andbottom terminal 330 decreased from its value of 1.2 MΩ after applicationof the first programming pulse described above to a value of 64 kΩ uponapplication of the second programming pulse. The decrease in themeasured resistance indicates that the programming pulse applied betweenintermediate terminal 370 and top terminal 390 induces a structuraltransformation in the chalcogenide material between the intermediateterminal 370 and bottom terminal 330 toward a more crystalline state.The occurrence of this structural transformation is an indication that aportion of the energy provided to the portion of chalcogenide materiallocated between the intermediate terminal 370 and top terminal 390 hasdiffused, been transported to or is otherwise dissipated into theportion of the chalcogenide material located between intermediateterminal 370 and bottom terminal 330. The structural transformationevidences the energy excursion effect described hereinabove whereinenergy provided to one portion of a chalcogenide material is transportedto and influences the structural state of another portion of thechalcogenide material. The transported energy is energy in excess ofthat needed to influence the structural state of the chalcogenidematerial between intermediate terminal 370 and top terminal 390.

The results of this aspect of the experiment demonstrate the potentialability of energy provided to one portion of a chalcogenide material toinfluence another portion of a chalcogenide material. If such energyexcursion is undesired, the experiment indicates that the amplitudeand/or duration of the second programming pulse should be decreased toavoid the production of the excess energy that is dissipated away fromthe portion of chalcogenide material that was intended to be programmed.Alternatively, the excess energy may be viewed as providing an extradegree of freedom in influencing the information content of portions ofthe chalcogenide material outside of the portion located between theterminals across which the programming pulse was applied. The structuralstate of the chalcogenide material between intermediate terminal 370 andbottom terminal 330 having a resistance of 64 kΩ may be viewed as aninformation state independent of and distinct from the set state(resistance in the vicinity of 8 kΩ) and reset state (resistance in thevicinity of 1 MΩ). Through control of the excess energy, it is possibleto continuously adjust the resistance of portions of a chalcogenidematerial outside of the portion directly programmed.

The instant invention includes memory cells having three or moreelectrical terminals in electrical communication with a volume ofchalcogenide material. The foregoing example considers an embodimenthaving three terminals. Corresponding embodiments having four, five ormore terminals are also contemplated. FIG. 5, for example, depicts anembodiment having four electrical terminals. The four terminal memorycell includes a pore filled with a chalcogenide material 510 that is incontact with a top electrical terminal 550, a bottom electrical terminal560, an upper intermediate electrical terminal 570 and a lowerintermediate terminal 580. An insulating or dielectric material ormaterials 590 separates the electrical terminals 550, 560, 570 and 580.Storage of data in the four terminal cell occurs by providing anelectrical signal to the chalcogenide material through any pair ofterminals. Since the device includes four terminals and since structuraltransformations of the chalcogenide material can be effected byproviding electrical energy through any two terminals, the four terminaldevice provides for multiple options for the programming ortransformation of the chalcogenide material and permits programming ofselected portions or regions of chalcogenide material in the pore of thedevice. The four terminal device provides for up to 3 multary bits ofstorage through the selective programming of up to 3 distinct portionsof the chalcogenide material in the pore depending on the combination ofterminals used in programming. One bit of storage is achievable byapplying an electrical signal between the bottom terminal 560 and thelower intermediate terminal 580 so that the lower portion 520 of thechalcogenide material in the pore is programmed to store a binary ornon-binary information value. A second bit of storage is achievable byapplying an electrical signal between the lower intermediate terminal580 and the upper intermediate terminal 570 so that the middle portion530 of the chalcogenide material in the pore is programmed to store abinary or non-binary information value. A third bit of storage isachievable by applying an electrical signal between the upperintermediate terminal 570 and top terminal 550 so that the upper portion540 of the chalcogenide material in the pore is programmed to store abinary or non-binary information value.

If desired, the four terminal device may also be used to store one ortwo bits of information through selective programming that involves lessthan all of the device terminals. Single bit programming, for example,can be achieved by applying an electrical signal between the topterminal 550 and the bottom terminal 560 so that programming occursacross the whole length of the chalcogenide material in the pore. Twobit programming may similarly be achieved through selective programmingthat utilizes any three of the four terminals. As one example of two bitprogramming of a four terminal cell, one may selectively program usingtop terminal 550, upper intermediate terminal 570 and bottom terminal560. Other combinations of three terminals may similarly be used toachieve two bit programming. Generalization to embodiments having morethan four terminals is readily apparent.

The foregoing examples have considered primarily embodiments having agenerally cylindrical pore with annular intermediate terminals. Otherembodiments of the instant invention include devices havingnon-cylindrical pores such as rectangular pores, spherical pores,channel pores, pores having a non-uniform cross-section etc. andelectrical terminals having shapes, including circumferential ornon-annular, suitable for providing electrical energy to non-cylindricalpores. The chalcogenide material is a continuous volume that occupies orfills the pore. An example of a three terminal device with a volume ofchalcogenide material in a pore having a non-uniform cross section isshown in FIG. 6. This three terminal memory cell includes a pore filledwith a chalcogenide material 610 that is in contact with a topelectrical terminal 620, a bottom electrical terminal 640, and anintermediate electrical terminal 630. An insulating or dielectricmaterial or materials 650 separates the electrical terminals 620, 630,and 640. In this embodiment, the intermediate terminal creates anarrowing of the pore diameter in the central portion of the pore toprovide non-uniformity in the diameter of the pore.

Also within the scope of the instant invention are embodiments in whichthe pore of the device includes two or more compositions of chalcogenidematerial. In a three terminal device, for example, the memory cellincludes a top terminal, a bottom terminal and an intermediate terminalwhich may be used to define separately programmable upper and lowerportions of chalcogenide material in the pore. A further embodiment ofthe instant invention provides for different chalcogenide compositionsin the upper and lower selectively programmable portions of the pore.Analogous embodiments are further provided for devices having four ormore terminals.

The electrical terminals of the instant devices are in electricalcommunication with the chalcogenide material. The electrical terminalsinclude a conductive material and are capable of providing electricalsignals such as electrical current pulses or voltage pulses to thechalcogenide material. The electrical terminals may directly orindirectly provide or induce current flow or a voltage gradient within achalcogenide material. Terminals that operate by injection or through afield effect are within the scope of the instant invention.Representative injection terminals include those described in co-pendingparent application U.S. patent application Ser. No. 10/384,994 andrepresentative field effect terminals include those described inco-pending parent application U.S. patent application Ser. No.10/426,321.

Methods of programming according to the instant invention are directedat the programming of chalcogenide memory cells having three or moreterminals in electrical communication with a chalcogenide material. Thethree or more terminals delineate separately programmable portions ofthe volume of chalcogenide material occupying the pore and the methodsof programming are directed at the storage of two or more binary ornon-binary bits or information values through the selective programmingof spatially distinguishable portions of chalcogenide material asdescribed hereinabove. The methods of programming include the steps ofselecting a pair of electrical terminals, providing an electrical signal(e.g. current pulse or voltage pulse) across the selected terminals sothat a portion of chalcogenide material between the selected terminalsis transformed to a structural state corresponding to an informationvalue that one wishes to store where the information value may be abinary or non-binary information value. Further programming can occur byselecting another pair of electrical terminals, applying an electricalsignal across those terminals so that another portion of chalcogenidematerial is programmed to store an information value. In this way,multiple multary bit storage is achieved in a continuous volume ofchalcogenide material within the pore of a memory cell. Various methodsof programming to achieve multiple multary bit storage are within thescope of the invention that include combinations of the steps ofselecting a pair of electrical terminals, programming across theselected pair of terminals by applying an electrical signal where theprogramming induces a structural transformation of a chalcogenidematerial to an information state corresponding to the information valuebeing stored, and repeating for other pairs of terminals.

The disclosure and discussion set forth herein is illustrative and notintended to limit the practice of the instant invention. While therehave been described what are believed to be the preferred embodiments ofthe instant invention, those skilled in the art will recognize thatother and further changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such changes and modifications that fall within the full scope ofthe invention. It is the following claims, including all equivalents, incombination with the foregoing disclosure, which define the scope of theinstant invention.

1. A chalcogenide device comprising: a chalcogenide material having aplurality of structural states, said structural states includingaccumulation states and greyscale states; a first terminal in electricalcontact with said chalcogenide material; a second terminal in electricalcontact with said chalcogenide material; a third terminal in electricalcontact with said chalcogenide material; a fourth terminal in electricalcontact with said chalcogenide material; wherein said chalcogenidematerial includes a first portion in a first structural state and asecond portion in a second structural state, said first and secondstructural states being selected from among said accumulation states orsaid greyscale states.
 2. The device of claim 1, wherein saidchalcogenide material comprises S, Sc, or Te.
 3. The device of claim 2,wherein said chalcogenide material further comprises Ge or Sb.
 4. Thedevice of claim 2, wherein said chalcogenide material further comprisesAs or Si.
 5. The device of claim 2, wherein said chalcogenide materialfurther comprises an element selected from the group consisting of Al,In, Bi, Pb, Sn, P, and O.
 6. The device of claim 2, wherein saidchalcogenide further comprises a transition metal.
 7. The device ofclaim 1, wherein the composition of said first portion of saidchalcogenide material differs from the composition of said secondportion of said chalcogenide material.
 8. The device of claim 1, whereinthe resistance of said first structural state differs from theresistance of said second structural state.
 9. The device of claim 1,wherein said first and second structural states are selected from amongsaid greyscale states.
 10. The device of claim 1, wherein said devicestores two or more bits of information.
 11. The device of claim 10,wherein said bits are non-binary bits.
 12. The device of claim 1,wherein said device stores three or more bits of information.
 13. Thedevice of claim 1, wherein said chalcogenide material has a shape havinga non-uniform cross section.
 14. The device of claim 1, furthercomprising one or more additional terminals in electrical communicationwith said chalcogenide material.
 15. The device of claim 13, whereinsaid chalcogenide material further includes a third portion in a thirdstructural state, said third structural state being selected from amongsaid accumulation states or said greyscale states.